1. Field of the Invention
This invention relates to improvements in an opto-magnetic retrieving system, in which data is recorded in and erased from an opto-magnetic memory with a laser beam while a magnetic field is applied to the memory, and also to a magnetic field generator assembled in the opto-magnetic retrieving system for generating a magnetic field.
2. Description of the Related Art
A data retrieving system utilizing an opto-magnetic retrieving method, there are external memories of computers, rewritable video disk systems and image file systems, DAD type compact disk systems capable of reproduction and recording and rewritable high density recording magnetic card system, these systems being recently developed. In such an opto-magnetic data retrieving system, a static magnetic field is applied perpendicularly to the recording surface of an opto-magnetic memory, i.e., a data recording medium, and a convergent laser beam is projected on the surface of a recording film of the data recording medium for heating the surface up to a temperature exceeding the curie point or a temperature, at which the coercive force is less than the external magnetic field to cause inversion of magnetic moments of domains in that area, thus effecting the recording or erasing of data. As a method of applying a magnetic field to the recording medium in such a system, there are a method, a magnetic field is applied locally only in the neighborhood of an area, which is illuminated by a convergent laser beam to form a beam spot, and an another method, in which a magnetic field is applied continuously over the entire region capable of illumination by the convergent laser beam, e.g., a region along the radius of opto-magnetic disk, on which the convergent laser beam can be shifted for accessing data.
In the prior art magnetic field generator adopting the method of locally applying magnetic field, an electromagnetic coil is wound around a core portion extending into a yoke as box-like magnetic flux returning section. In this magnetic field generator, end surfaces of the core and the yoke oppose the recording film surface of the opto-magnetic memory. When a current is supplied to the electromagnetic coil, a magnetic flux applied from the end surface of the core is spread as it penetrates the recording film surface of the opto-magnetic memory and returns to the end surface of the yoke surrounding the end surface of the core. Thus, a magnetic field perpendicular to the recording film surface of the opto-magnetic memory, is applied locally to the recording film surface. Where the local magnetic field application method is adopted, the magnetic field generator itself is formed to have a small size with the longitudinal dimension of the core and the yoke end surfaces of the magnetic field generator set to the length of a local area, to which magnetic field is to be applied.
On the other hand, in a prior art magnetic field generator adopting the another, in which a magnetic field is applied over the entire data retrieval area of the opto-magnetic memory, an electromagnetic coil is wound around an elongated core portion, which extends into and substantially parallel to a yoke having a U-shaped cross sectional profile and serving as magnetic flux returning path. In this magnetic field generator, parallel end surfaces of the core and the yoke oppose the recording film surface of the opto-magnetic memory. When a current is supplied to the electromagnetic coil, a magnetic flux supplied from the end surface of the core penetrates the recording film surface of the opto-magnetic memory to return to the end surface of the yoke. Likewise, a magnetic field perpendicular to the recording surface of the opto-magnetic memory, is applied to the entire area of the recording film surface capable of laser beam illumination, i.e., area capable of data retrieval. When the method of applying a magnetic field to the entire area is adopted, the core and the yoke end surfaces of the magnetic field generator have substantially equal width and a substantially equal length, and the longitudinal dimension of the core and the yoke end surfaces is set to be greater than the length of the opto-magnetic recording surface of the opto-magnetic memory, so that the magnetic field generator itself has a comparatively large size.
In the prior art magnetic field generator, which adopts the local magnetic field application method, a magnetic field is applied locally. Therefore, the magnetic field generator itself has to be moved with the movement of the laser bean:. Thus, the magnetic field generating system requires a complicated moving mechanism. On the other hand, the prior art magnetic field generator, which adopts the method of applying a magnetic field to the entire area, although it does not require movement of the magnetic field generator, has the following problems.
(1) The power consumption is high. If a magnetic field of high intensity has to be applied to the data recording medium, a high magnetomotive force has to be generated from the electromagnetic coil. This magnetomotive force is determined by the product N.times.I of the total turns number N of the electromagnetic coil and current I supplied to the coil. The power P consumed in the coil is EQU P=I.times.R.sup.2
where R is the resistance of the coil. In the magnetic field generator, the longitudinal dimension of the core portion is determined in correspondence to the length of the recording surface of the opto-magnetic recording medium, and hence the outer periphery thereof has a comparatively great length. This means that the conductor wire of the N-turn electromagnetic coil wound around the elongated core portion has an extremely great total length. Therefore, the resistance R of the coil is correspondingly high, and the necessary power consumption is also correspondingly high. If the size of the conductor wire is increased to reduce the resistance R of the electromagnetic coil, the turns number N is reduced with a predetermined size of the electromagnetic coil accommodation space. Further, if a predetermined magnetomotive force is to be obtained with reduced turns number N, a large current has to be supplied to the coil. This again means a power consumption increase.
(2) Heat is greatly generated. When the power consumption is high, the heat generation is increased as noted above. The electromagnetic coil, which is a heat-generating part of the magnetic field generator, and the core and yoke, to which the heat from the coil is transmitted, are disposed sufficiently closely to and oppose the opto-magnetic recording medium. Therefore, it is liable that heat transmitted from the magnetic field generator to the opto-magnetic recording medium causes thermal deformation of the opto-magnetic memory or loss of recorded data.
(3) The size of the magnetic field generator is increased; particularly, the height of the magnetic field generator in the direction perpendicular to the recording surface is increased. If the flux density on the recording surface is to be increased, it is necessary to sufficiently reduce the width of the core and sufficiently increase the height of the core and yoke so as to arrange the turns of the coil in a direction perpendicular to the recording surface. With such a structure, the height of the magnetic field generator is inevitably increased. Where a magnetic field generator having a large height is disposed above the opto-magnetic recording medium, the data retrieving system itself inevitably has a large size. This means a restriction imposed on the design of the system.
The problems of size increase of the data retrieving system are presented not only in the magnetic field generator utilizing an electromagnet but also in the magnetic field generators utilizing permanent magnets as disclosed in Japanese Patent Laid Open Disclosures Nos. 59-54,003, 62-8,345 and 62-14,352. That is, it is difficult to reduce the size of these magnetic field generators.